Recently, camera modules are mounted on smart phones, automobiles, medical devices, and so on. With the developments of technology, the resolution of the camera increases, while the size of the camera reduces significantly. The camera modules are manufactured by using typically complementary metal oxide semiconductor (CMOS) image sensors. A CMOS image sensor comprises a plurality of pixels.
Standard image sensors have a limited dynamic range of approximately 60 to 70 dB. However, the luminance dynamic range of the real world is much larger. Natural scenes often span a range of 90 dB and over. In order to capture the highlights and shadows simultaneously, high dynamic range (HDR) technologies have been used in image sensors to increase the captured dynamic range. A technique to increase dynamic range is to merge two exposures captured with standard (low dynamic range) image sensors into a single linear HDR image, which has much larger dynamic range than a single exposure image.
The two exposures may be captured using two different gains known as dual conversion gain (DCG). DCG includes high conversion gain (HCG) and low conversion gain (LCG). LCG may be provided by an added capacitor in the pixel circuit. The area of the capacitor is limited due to the small pixel size. Thus its capacitance is also limited. This in turn limits the dynamic range. Accordingly, a higher dynamic range is demanded, while the size of the capacitor is unchanged.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention.